Inside a CFL bulb – Less power, longer life

As more and more states in America begin the process of phasing out the traditional incandescent bulb, our attention turns to the changing lighting landscape. It’s a landscape that includes lighting alternatives that offer less energy consumption, better durability and a longer shelf-life than the Edison-devised model that has been the “go-to” for light for the better part of a century. The reduced energy consumption also saves consumers money– which is always appealing.

Two of those alternatives that are beginning to reach mass market appeal are the compact fluorescent lamp (CFL) and the LED lamp. Both offer the positive benefits mentioned above but for the purpose of this teardown, we will be focusing on CFL technology. In particular, we will be looking at a non-dimmable CFL demonstration lamp made by Chinese manufacturer, Baishi.

CFL Technology – 35 years in the making
The spiral tube CFL was first introduced in 1976 by Edward E. Hammer, considered by many to the father of modern fluorescent lamp technology. Though the design was a great improvement over the incandescent bulb, his employers at General Electric shelved the design. The first introduction of a lamp with an electronic ballast was made by Osram in 1985, but it wasn’t until 1995 that CFL bulbs began commercial production.

Examining what makes this lamp at 20-Watts produce the same illumination as a 75-W incandescent bulb requires a look at each component that comprises the lamp. Like many of its fluorescent lamp cousins, this CFL uses U-shaped glass tubes whose phosphor lining emits from UV radiation manufactured by ionized gas in the tube.

To achieve a size that would be primed for the housing market, CFLs exhibit a design that typically use a smaller tube diameter and/or through folded or spiral-wound construction. The electronic ballast, that is used to light the tube, is designed into a manner that remedies the space constraints of a standard screw-into-socket bulb. To do this, a downsized supply is used to create a suitable lamp drive in relatively small footprint of area.

The phosphor-coated U-shaped glass tube has its own evolved history in manufacturing but that is a history more suited for chemical engineering buffs. From the perspective of the consumer, wearing out of lamp phosphors within the tube remains the primary issue. Still, the lifetime of quality CFLs can run from months to even years.

Ballast issues
Also of note by looking at this particular CFL, the occurrence of vent holes in the lamp-base enclosure. This suggests that heat-related wear out of ballast electronics are also a factor.

The typical electronic ballast consists of a small circuit board and it is this board that allows consumers to replace incandescent bulbs easily with CFL alternatives. The electronic ballast provides the initial boost of energy to start the lamp and then limits the current to its operating value for the tube that is being used.

Transistors, diodes, and collection of capacitors and inductors are placed into the ballast design to first rectify the AC power and create a follower resonant inverter circuit that generates the high-frequency, high-voltage power used to fire the lamp. Within this electronic ballast, one of the most important components is UBA2211 Half-Bridge power IC from NXP.

The UBA2211 Half-Bridge is capable of driving lamps up to 25 W from 110 or 220 V-AC input, necessary in any electronic ballast found in modern CFLs. One of the prime benefits of the UBA2211, however, is the inclusion of OverTemperature Protection (OTP) and Capacitive Mode Protection (CMP) within the circuit. OTP and CMP monitor the voltage, current and power dissipation and, in non-standard conditions, ensure correct system shutdown and safe conditioning at the burner during end-of-life. During the initial “kick” of power during the CFL’s start-up, Saturation Current Protection (SCP) is also provided by the UBA2211 so that the CFL is capable of operating at the saturation current limit without potentially damaging the power transistors in the electronic ballast.

CFL – A “green” solution in more ways than one
This article didn’t answer the age old question of “How many engineers does it take to screw in a light bulb?” but it did answer how many engineers does it take to take one apart. In the case of the CFL, intricate power circuitry is revealed when we do take it apart which also reveals how it has gotten to a place within the market that the combination of lamp and electronic drive circuits can now be brought to general consumers at prices that start to rival the (relatively) short-lived incandescent alternative.

Pricing differences still exist – a typical incandescent bulb costs only a quarter where CFLs still neighbor in the $1 to $2 range – but the real advantage comes from the potential energy savings. Though the incandescent bulb is 25 cents, the cost to operate that bulb over its average lifetime is $7. The CFL bulb, through its lifetime (a much longer lifetime), presents a cost of only a $1.50. In that sense, not only is the CFL bulb a “green” solution from the perspective of reducing energy, but also a “green” solution for keeping more green in your wallet.

I had incandescents in every room in the house with the exception of the kitchen and family room where we had conventional fluorescence. we had been in the house 8 years and changed only one incandescent globe. About 4 years ago we were given a bunch of GE CFL's to replace the standard size incandescents and have replaced 3 of them already. Given the increased price of them and the greater carbon footprint I would suggest that CFL's only make financial and environmental sense in locations where they run for hours at a time, because they certainly DO NOT last longer in terms of time since first turn on as our experiment has shown. I believe the CFL industry has pulled the wool over everyone's eyes on this one. As they say, the proof of the pudding.....

I agree with you, and preheating increases life of tube considerately but it also consumes more energy, in a CFL, a few milliwats plus the added cost...and in buildings, where common areas are lit by CFLs, the PFC becomes very important because it adds up to a lot of noise and distortion, but I haven´t seen the first CFL witha PCF controller

It would be interesting to know what issues this IC was intended to address, save for IC-controlled inductor saturation. Few commodity CFL incandescent lamp replacements actually use an IC, while demonstrating operation over a considerably wider operating input voltage range, without apparent resonant inductor saturation issues. Perhaps this is more of an issue at 240VAC?
Does the IC allow cooler operation? Generate less noise or distortion? Avoid the use of high voltage electrolytic or low temperature film capacitors? Cost less? Use mercury-free tubes? Allow hands-free assembly? Has NXP missed the boat on this one?

There are 4 major things (problems) that are basic for CFLs:
1. Colour - so called light temperature should always be below 4000K (nice sunny colour for domestic use) unless you're a fan of horror movies.
2. Warm start - if you want to enjoy your energy efficient light for long make sure you buy one with this feature. They're easy to spot - the warm start ones take longer to light up (about 1 sec) and the filaments light up first. This allows the gas to be heated inside to lower the ignition voltage. Most of the CFLs on the market DO NOT have it - manufacturers are sparing 30cc PTC posistor.
3. Position - if you place your CFL upside down it will live shorter because of the temperature - hot air tends to go up and if the electronics PCB is higher then the light tube then it heats more. The last CFLs I bought have even a warning on the box.
4. All (to my knowledge) CFLs at the market now DO NOT have any power factor correction therefore they draw non-sinusoidal highly distorted current (spikes with amplitude 5 times higher then RMS value). That is normally not a problem for a domestic lighting but may be a problem in a big building (neutrals wires overheating in 3 phase system) and makes the grid a bit less effective. Fortunately power drawn by CFLs (domestic) are just a small percentage of the whole power to make it a significant problem.